Field of the Invention
The present invention relates to device and method for calculating an amount of drift. The invention also relates to a charged particle beam system.
Description of Related Art
In a scanning electron microscope (SEM), a scanning transmission electron microscope (STEM), or other similar instrument, if a drift of a sample occurs, then a second scan results in an image of an observed subject deviating from an image of the subject produced by a first scan.
This drift of the sample is corrected, for example, by preparing a reference image providing a reference, creating an image of interest (image to be compared) that has drifted during an observation or analysis, calculating a cross-correlation function or a phase-only correlation function from the reference image and from the compared image, computing the amount of deviation (amount of drift) of the compared image from the reference image, and feeding the computed amount back to the apparatus. The amount of drift corresponds to a relative position between the position where the cross-correlation function or phase-only correlation function produces a maximum intensity and the center of the function.
FIG. 13 is a flowchart illustrating one example of a general process for correcting drifts. This process includes step S10 for obtaining a reference image, step S12 for performing an observation or an analysis, step S14 for obtaining an image to be compared against the reference image, step S16 for computing an amount of drift, and step S18 for correcting the amount of drift by feeding information about the computed amount of drift to an electron microscope. If the decision at step S12 is negative (No), indicating that neither an observation nor an analysis is performed, then the process is ended.
FIG. 14 is a flowchart illustrating one example of the step S16, i.e., a general process for computing an amount of drift. This process includes step S162 for taking Fourier transforms of a reference image and an image to be compared, step S164 for calculating a cross-correlation function between the reference image and the compared image, and step S166 for searching for a position at which the cross-correlation function shows a maximum intensity.
Where an image of the subject of observation or analysis shows a periodic structure such as an atomic arrangement, it is difficult to judge, using the cross-correlation function, whether the amount of the deviation of the compared image from the reference image is one period of the periodic structure or exceeds one period. Where the actual drift is less than one period, the amount of drift may be calculated to be greater than one period using the cross-correlation function, for example, due to image blurring or by the effects of noise.
FIG. 15 schematically shows an image D in which a reference image containing a periodic structure and a compared image containing a periodic structure are superimposed. In FIG. 15, small circles with leftwardly and downwardly drawn hatching lines indicate particles of the reference image. Small circles with rightwardly and downwardly drawn hatch lines indicate particles of the compared image. In the overlapped image D shown in FIG. 15, there is no drift between the reference image and the compared image. However, the reference image and the compared image suffer from image blurring, and the period deviates in some locations.
FIG. 16 shows one example of a cross-correlation function between a reference image having blurred portions and a compared image having blurred portions. An indicia “x” shown in FIG. 16 indicates a position where a maximum intensity occurs. In FIG. 16, no drift occurs between the reference image and the compared image.
Where neither the reference image nor the compared image is blurred, the amount of drift is zero, and the center of the cross-correlation function exhibits a maximum intensity. However, where the reference image and the compared image suffer from image blurring, the position at which the cross-correlation function shows a maximum intensity deviates from the center although there is no drift as shown in FIG. 16. For example, in the example shown in FIG. 16, the position at which a maximum intensity occurs deviates one period from the center in the X-axis direction although there is no drift between the reference image and the compared image. In this case, the cross-correlation function suffers from a deviation of one period by the effects of image blurring in spite of the fact that there is no drift. Consequently, the drift is overcorrected.
In view of this problem, JP-A-2013-165003 proposes a solution. That is, if a pattern contained in plural image signals is judged to have periodicity when a matching process is performed between the image signals, the image signal region to be processed is narrowed down to a region where two or more patterns of identical geometry are not contained. Consequently, even if an image including a repetitive pattern is obtained, an appropriate accumulation signal can be formed.